Field of the Invention
The invention relates to a method and a device for determining the absolute angle of rotation of an object that is rotating about an approximately horizontal rotational axis, in particular a motor vehicle rotating about its longitudinal axis.
In modern occupant protection systems for motor vehicles, the task arises, in the event of a rollover about the longitudinal axis, of detecting the absolute angle of rotation of the vehicle in order to be able to correctly time a decision to fire a restraining device or other protection devices.
Issued European Patent No. EP 0 430 813 B1 discloses a safety system for motor vehicles that operates with four sensors. A rate of rotation sensor detects the angular velocity of the rotation of the vehicle about its longitudinal axis. Three acceleration sensors detect the longitudinal acceleration, the transverse acceleration, and the vertical acceleration of the vehicle. The output signal of the rate of rotation sensor is integrated, and if it is above a threshold value, is fed to an OR element so that a pyrotechnic firing device is tripped. The integration of the output signal from the rate of rotation sensor is not performed continuously, but only during a time window that is determined by the output signals from the acceleration sensors.
Published European Patent Application EP 0 934 855 A1 discloses a method and a device for tripping a rollover protection device in which longitudinal, lateral, and vertical vehicle accelerations as well as rollover and pitching ratesxe2x80x94supplied by corresponding sensorsxe2x80x94are subjected to time averaging. The time-averaged signals are subjected to an expanded Kalman filter. Output variables of the Kalman filter is the current rollover angle and the current pitch angle of the vehicle. All five input variables must be fully utilized in order to calculate each of these variables.
It is accordingly an object of the invention to provide a method and a device for determining the absolute angle of rotation of an object that is rotating about an approximately horizontal rotational axis, in particular a motor vehicle rotating about its longitudinal axis. Using the method and the device it is possible to determine the absolute angle of rotation quickly and accurately in conjunction with simple executability and simple design.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for determining an absolute rotational angle of an object that is rotating about an approximately horizontal rotational axis. The method includes steps of: determining a change in a rotational angle of the object occurring during a time interval; determining an acceleration component acting in a direction of a vertical axis of the object; determining a change, occurring during the time interval, in the acceleration component acting in the direction of the vertical axis of the object; and calculating the absolute rotational angle of the object from the change in the rotational angle and the change in the acceleration component acting in the direction of the vertical axis of the object.
In accordance with an added mode of the invention, the object is a motor vehicle rotating about a longitudinal axis thereof.
In accordance with an additional mode of the invention, the method includes: calculating the absolute rotational angle using the following formula:
xcex1=(m)(xcex94azxe2x88x92xcex94az0), 
where xcex1 is the absolute rotational angle, and
where m=C3/xcex94xcex1, xcex94az0=1xe2x88x92cos (xcex94xcex1), C3 is a constant, xcex94xcex1 is the change in the rotational angle, and xcex94az is the change in the acceleration component.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for determining an absolute angle of rotation of an object that is rotating about an approximately horizontal rotational axis. The device includes: a rate of rotation sensor for detecting an angular velocity of the object rotating about the rotational axis; an integrator for integrating the angular velocity and for determining a change in an angle of rotation; an acceleration sensor for detecting an acceleration component acting in a direction of a vertical axis of the object; a difference element for determining a change, accompanying the change in the angle of rotation, in the acceleration component acting in the direction of the vertical axis; and an arithmetic unit for calculating the absolute angle of rotation of the object from the change in the angle of rotation and the change in the acceleration component.
In accordance with an added feature of the invention, there is provided a high-pass filter configured between the rate of rotation sensor and the integrator.
In accordance with an additional feature of the invention, there is provided a high-pass filter configured between the acceleration sensor and the difference element.
In accordance with another feature of the invention, the arithmetic unit operates using the following formula:
xcex1=(m)(xcex94azxe2x88x92xcex94az0), 
where xcex1 is the absolute rotational angle, and
where m=C3/xcex94xcex1, xcex94az0=1xe2x88x92cos(xcex94xcex1), C3 is a constant, xcex94xcex1 is the change in the rotational angle, and xcex94az is the change in the acceleration component.
In accordance with a further feature of the invention, the arithmetic unit is programmed to perform the following steps:
A) detect xcfx89(t) and az(t), where t=time, xcfx89(t) is the angular velocity, and az(t) is the acceleration component;
B) determine             Δα      ⁢              (        T        )              =                  ∫        0        T            ⁢                        ω          ⁢                      (            t            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          t                      ,
where T=a time interval, and calculate xcex94az=az(T)xe2x88x92az(0);
C1) if xcex94xcex1 less than C1, where C1 is a constant, then proceed to step A),
C2) if xcex94az greater than C2, where C2 is a constant, then proceed to step A; and
D) calculate:
xcex1=mxc2x7(xcex94azxe2x88x92xcex94az0), where 
m=C3/xcex94xcex1, xcex94az0=1xe2x88x92cos (xcex94xcex1) 
and C3 is a constant.
According to the invention, the change, caused by gravity, in the acceleration in the direction of the vertical axis of the vehicle, and the angle by which the vehicle rotates around its longitudinal axis are determined within a time interval. The two items of information can be used to calculate the absolute angle of rotation of the vehicle, that is to say its absolute rotary position. Because only differences of signals are considered, absolute sensor values are not necessary, and so no expensive, high-stability acceleration sensors need to be used. Instead, sensors of simple design can be used.
The invention can advantageously be applied wherever the aim is to determine the absolute angular position of an object that is rotating, in a gravity field, around a rotational axis inclined to the direction of gravity. The invention takes advantage of the fact that the gravity component or acceleration component changes in accordance with a sinusoidal or cosinusoidal function in the direction of an axis that is fixed in the object and that is rotating relative to the direction of gravity during the rotation, so that it is possible from the change in the gravity component and in the angle of rotation of the object to reach a conclusion on the absolute angle of rotation and the rotary position of the latter in a fixed coordinate system. The invention is particularly suitable for use in motor vehicles for the purpose of expediently tripping safety devices that protect against the consequences of a rollover.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for determining the absolute angle of rotation of an object that is rotating about an approximately horizontal rotational axis, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.